The present invention relates to phase-conversion surround circuitry for sound-field reproduction used in audio system.
Audio systems for music-data reproduction from cassette tapes, CDs, MDs, memory cards, DVDS, etc., are usually equipped with a digital surround circuit or an analog phase-conversion (shift) surround circuit, for offering surround effects to create wide sound fields.
Shown in FIG. 1 is a known analog phase-conversion surround circuit 20 equipped with phase-delay units 4 and 5.
Input right- and left-channel audio signals R and L are supplied to two-stage phase (90xc2x0)-delay units 4 and 5 via low-frequency amplifiers (operational amplifiers) 1, 2 and 3.
A resultant 180xc2x0-delayed signal xcex94(Rxe2x88x92L) is combined (added/subtracted) with the right- and left-channel audio signals R and L at low-frequency amplifiers (operational amplifiers) 7 and 8, respectively, via an ON/OFF switch SW and an allotter 6, thus composite signals R+xcex94(Rxe2x88x92L) and Lxe2x88x92xcex94(Rxe2x88x92L) being output.
Three or more stages of the phase-delay units for longer delay time will enhance surround effects (wider sound fields). Nevertheless, the more enhanced surround effects, the more difference in sound quality based on addition/subtraction-resultant composite signals, so that most listeners feel low sound quality. For example, four-stage phase-delay units for 360xc2x0-delay could cause that most people cannot listen for a long time. On the contrary, one or two-stage phase-delay units for less delay act like an equalizer and reduce surround effects.
Moreover, the known phase-conversion surround circuit 20 gives a fixed level of surround effects, thus the created sound being not feasible for healing effects with BGM, quiet music, music with full of low-level sounds, etc.
A purpose of the present invention is to provide a phase-conversion surround circuitry with enhanced surround and healing effects.
The present invention provides a phase-conversion surround circuitry to give surround effects to right- and left-channel audio signals, including: a phase delayer to delay an audio difference signal between the right- and left-channel audio signals, thus outputting a delayed audio difference signal; a combiner to combine the delayed audio difference signal with the right- and left-channel audio signals, thus outputting composite audio signals; and a controller to oscillate at frequency of 12 Hz or less to periodically vary a time constant of the phase delayer.